The present invention relates to an ultrasonic motor, particularly, to an ultrasonic motor of which rising characteristics at start-up are improved.
An ultrasonic motor is configured with a stator, which includes a piezoelectric body with a plurality of polarized piezoelectric segments circumferentially arranged, and a rotatable disc-shaped or annular rotor in contact with the stator under a predetermined pressure. In the ultrasonic motor, the piezoelectric body of the stator is vibrated with radio frequency voltage being applied thereto. The induced vibration of the piezoelectric body is enhanced in a circumferential direction of the stator by a comb body provided integrally to the piezoelectric body such that the comb body is driven to induce a traveling vibration wave in the circumferential direction. Thereby, the rotor, which frictionally engages with the piezoelectric body, can be rotated around an axis thereof. For example, the above features are disclosed in Japanese Patent Provisional Publication No. 2000-60154. The comb body has a function of enlarging amplitude of the vibration of the piezoelectric body. However, since the amplitude is generally one micrometer to three micrometers, it is required that the comb body establishes close contact with the rotor evenly in the circumferential and radial directions of the stator, and that both close-contact surfaces between the comb body and the rotor are configured as pressure contact surfaces with a desired pressure contact force being applied thereto, so as to improve rotational efficiency of the rotor (i.e., rotational energy of the rotor to vibration energy of the stator). Therefore, when the ultrasonic motor has not been driven for a long time, the pressure contact surfaces between the comb body and the rotor come into a state of interfacial adhesion due to the pressure contact force, and it increases a static frictional force therebetween. Thereby, a large torque is needed for rotating the rotor at start-up of the ultrasonic motor, and the ultrasonic motor cannot begin smooth rotation, that is, the rising characteristics of the ultrasonic motor at the start-up is worsened. In the worst case, the motor cannot be rotated.
In order to solve such a problem at the start-up of the motor, there has been proposed a technology in which a resin layer with a low frictional coefficient is formed on any of the pressure contact surfaces between the comb body and the rotor to reduce the static frictional force therebetween. Fluorocarbon resin such as polytetrafluoroethylene (PTFE) can be cited as an example of the resin layer. In Japanese Patent Provisional Publication No. HEI 9-98587, there is proposed a technique in which a slider formed from polymer resin is attached onto a surface of the rotor. The technique is regarded as one of possible solutions that can prevent the interfacial adhesion between the pressure contact surfaces of the comb body and the rotor and reduce the static frictional force. However, in this kind of resin, when temperature rises along with the rotation of the motor, the frictional coefficient thereof decreases, and both the pressure contact surfaces between the comb body and the rotor come into a slippery state. Hence, the traveling vibration wave of the comb body cannot efficiently be transmitted to the rotor, and thereby the rotational efficiency of the ultrasonic motor gets worse.
Thus, the resin layer with a low frictional coefficient formed on one of the pressure contact surfaces between the comb body and the rotor might cause the worsened rotational efficiency of the ultrasonic motor, and is not necessarily useful. In view of the above problem, there can be proposed a technique in which, instead of forming the resin layer, a high voltage is applied to the ultrasonic motor or the frequency of a radio frequency voltage is made higher only at the start-up, increasing a torque of rotation torque so as to release the adhesion. However, it is necessary for releasing the adhesion in a static state to apply a very large rotation torque to between the comb body and the rotor. Therefore, the voltage or the frequency to be applied to the ultrasonic motor is required to be very high. This is because, similarly to the case of the static frictional coefficient, a force for releasing an adhesion state where two substances are adhered to each other in their static states is larger than a force for releasing an adhesion state where the two substances are adhered to each other in their micro-vibrating states. Hence, a high voltage generating circuit configured to generate a very high voltage has to be employed as a driving circuit of the ultrasonic motor, and it results in an intricate and expensive driving circuit. In addition, since a temporarily-applied high voltage may cause a damage of the piezoelectric body, applying the high voltage to the ultrasonic motor each time starting up the motor may lead to rapid deterioration of the motor and thereby to a short life of the motor. Further, the ultrasonic motor begins to rotate at a high speed immediately after the adhesion state is released in a state where the high voltage is being applied, and a shock vibration and/or impact noise that may be generated at that time is a significant problem.